Trigonostigma heteromorpha: A Comprehensive Guide

Trigonostigma heteromorpha, commonly known as the Harlequin Rasbora, is a popular freshwater fish among aquarists for its striking coloration and peaceful nature. This guide covers everything you need to know about Trigonostigma heteromorpha, from their natural distribution and habitat to their dietary needs and spawning behavior.

Distribution and Habitat

Trigonostigma heteromorpha is native to the slow-moving waters of Southeast Asia, specifically in Thailand, Malaysia, and Singapore. They are commonly found in peat swamps and forest streams, where the water is soft, acidic, and stained with tannins from decaying vegetation.

Size and Appearance

Harlequin Rasboras are small, typically growing to about 2 inches (5 cm) in length. They have a distinctive black triangular patch on their flanks, set against a shimmering coppery-orange body. This striking contrast makes them a favorite in planted aquariums.

Water Parameters

Maintaining the right water conditions is essential for the health of Trigonostigma heteromorpha:

  • Temperature: 72-80°F (22-27°C)
  • pH: 5.0-7.0
  • Hardness: 1-5 dGH

These fish thrive in soft, slightly acidic water and benefit from a well-planted aquarium that mimics their natural habitat.

Tank Mates

Harlequin Rasboras are peaceful and social fish, making them ideal for community tanks. Suitable tank mates include:

  • Other small, peaceful rasboras
  • Tetras
  • Corydoras catfish
  • Dwarf gouramis
  • Shrimp

Avoid housing them with larger, aggressive species that might intimidate or prey on them.


In the wild, Trigonostigma heteromorpha feed on small insects, crustaceans, and zooplankton. In captivity, they are omnivorous and thrive on a varied diet:

  • High-quality flake food
  • Micro pellets
  • Live or frozen foods such as daphnia, brine shrimp, and bloodworms

A balanced diet ensures vibrant coloration and overall health.


Harlequin Rasboras are active and schooling fish, best kept in groups of at least six. This grouping helps reduce stress and encourages natural behaviors. They are generally peaceful and make excellent inhabitants for planted community tanks.


Breeding Trigonostigma heteromorpha in captivity can be achieved with the right conditions. To spawn them successfully:

  1. Conditioning: Feed the breeding pair a high-protein diet, including live foods, for several weeks.
  2. Breeding Tank: Set up a separate breeding tank with soft, acidic water and a temperature around 78°F (26°C). Include broad-leaved plants, as they are preferred spawning sites.
  3. Spawning Process: The female will lay eggs on the underside of leaves. Remove the adults after spawning to prevent them from eating the eggs.
  4. Hatching: Eggs typically hatch within 24-36 hours. Feed the fry infusoria or liquid fry food until they are large enough to consume baby brine shrimp.

Fun Facts

  • Trigonostigma heteromorpha is often referred to as the "Harlequin Rasbora" due to the harlequin-like black patch on its body.
  • They are known for their schooling behavior and are often seen swimming in tight formations, creating a mesmerizing display.
  • Harlequin Rasboras are hardy and adaptable, making them excellent choices for both novice and experienced aquarists.


Trigonostigma heteromorpha, with their vibrant coloration and peaceful nature, make a captivating addition to any freshwater aquarium. By understanding their natural habitat, dietary needs, and breeding behavior, aquarists can ensure these beautiful fish thrive in captivity. Whether you're a beginner or a seasoned fish keeper, the Harlequin Rasbora offers a delightful glimpse into the world of small freshwater fish.